Molecular basis for membrane selectivity of an antimicrobial peptide, magainin 2

Interactions of an antimicrobial peptide, magainin 2, with lipopolysaccharide-containing liposomes as a model for outer membranes of gram-negative bacteria

F12W-magainin 2 preferentially interacted with lipopolysaccharide-containing bilayers, permeabilizing the membranes, compared with lipopolysaccharide-free phosphatidylcholine vesicles. Using this system, we demonstrated for the first time that the magainin peptide forms a helix upon binding to lipopolysaccharide. Incorporation of lipid A into phosphatidylcholine liposomes also enhanced interactions with the peptide. The presence of Mg2+, which nullifies the peptide's antibacterial activity against gram-negative bacteria, again weakened the interactions between the peptide and lipopolysaccharide-doped bilayers. This system seems to be useful for investigating the molecular details of peptide-lipopolysaccharide interactions.

Gaegurin 4, a peptide antibiotic of frog skin, forms voltage-dependent channels in planar lipid bilayers

Gaegurin 4 (GGN4) is a cationic peptide of 37 amino acids (MW 3748) isolated from the skin of Rana rugosa. It has shown a broad spectrum antimicrobial activity in vitro against Gram-negative and -positive bacteria, fungi and protozoa. To understand its mechanism of antimicrobial action, we examined the effect of GGN4 on the membrane conductance and the electrical properties of GGN4-induced pores in planar lipid bilayers under voltage clamp. Natural and synthetic GGN4 (0.01-1 microg/mL) increased the membrane conductance in a concentration-dependent manner, but GGN4 (1-23), an N-terminal fragment of the peptide with little antimicrobial activity, failed to increase the conductance. At symmetrical 100 mM KCI, unitary conductances of about 120 pS were frequently observed. Their current-voltage relations were linear and open state probabilities were close to 1, but longer closing events were seen more frequently at negative voltages. In addition, GGN4-induced pores were selective for cation over anion, the permeability ratio of K+ to Cl- being 6: 1 in neutral and 7: 1 in acidic lipid bilayers. In conclusion, our results indicate that GGN4 forms voltage-dependent and cation-selective pores in planar lipid bilayers. The ionophoric property of GGN4 is likely to contribute to its antimicrobial activity.

Structure-antibacterial, antitumor and hemolytic activity relationships of cecropin A-magainin 2 and cecropin A-melittin hybrid peptides

In order to elucidate the structure-antibiotic activity relationship of cecropin A-magainin 2 and cecropin A-melittin hybrid peptides, several truncated peptides and the analogues with amino acid substitutions were synthesized and their antibacterial, antitumor and hemolytic activities of were examined. Cecropin A-magainin 2 hybrid analog, L16-CA(1-8)-MA(1-12) (termed as L-CA-MA in this study: KWKLFKKIGIGKFLHLAKKF-NH2), is known to have potent antibacterial and antitumor activity with less hemolytic activity. We found that the C-terminal region of L-CA-MA is more involved in the alpha-helical structure on cell membrane-like environment than N-terminal one by circular dichroism analysis. Deletion of the Gly-Ile-Gly sequence, the central hinge region of L-CA-MA, produced a considerable reduction in antitumor and hemolytic activity rather than an antibacterial one. The insertion of Pro, Gly-Ile or Gly-Pro in this hinge region of L-CA-MA caused retention of both antibacterial and antitumor activity while causing a significant decrease in hemolytic activity. However, the substitution with Gly-Pro-Gly instead of the Gly-Ile-Gly in CA(1-8)-MA(1-12), CA(1-8)-ME(1-12), CA(1-13)-MA(1-13) and CA(1-13)-ME(1-13) hybrids resulted in a drastic decrease in antibacterial, antitumor and hemolytic activity. The increase of hydrophobicity at position 16 in CA(1-8)-MA(1-12) by substituting Trp or Phe induced a significant increase in hemolytic activity without a considerable change in either antibacterial or antitumor activity. Therefore, these results suggested that the appropriate flexibility in the hinge region of CA-MA and CA-ME hybrid peptides and the appropriate hydrophobicity at position 16 in the hydrophobic region of CA (1-8)-MA(1-12) are important in potent antibacterial and antitumor activity with no hemolytic effect.

Antimicrobial peptides from the Brazilian frog Phyllomedusa distincta

Different peptides were purified by chromatographic procedures from the skin-secretory glands of the frog Phyllomedusa distincta. These are the first peptides reported from this frog species. Their primary structure was determined by a combination of automated Edman degradation and mass spectrometry. Peptide Q2 contains 25 amino acid residues, peptide Q1 and L have 28 each, peptide M contains 31, and peptide K has 33 amino acid residues. They all showed potent antimicrobial activity against Gram-negative and Gram-positive bacteria, presenting minimal inhibitory concentrations from 0.6 to 40 microM, when tested against Enterococcus faecalis, Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa. Peptides K, L, and Q1 were chemically synthesized and shown to be active.

Lipid polymorphism and protein-lipid interactions

Non-lamellar-forming lipids play an important role in determining the physical properties of membranes. They affect the activity of membrane proteins and peptides. In addition, peptides which lyse membranes as well as those which promote membrane fusion facilitate the formation of non-lamellar phases, either micelles, cubic or hexagonal phases. The relationship of these diverse effects on membrane curvature is discussed in relation to the function of certain peptides and proteins. Specific examples of ionophoric peptides, cytotoxic peptides and viral fusion peptides are given. In addition, we compare the modulation of the rate of photoisomerisation of an integral membrane protein, rhodopsin, by non-lamellar-forming lipids with the effects of these lipids on an amphitropic protein, protein kinase C. Among these diverse systems it is frequently observed that the modulation of biological activity can be described in terms of the effect of the peptide or protein on the relative stability of lamellar and non-lamellar structures.

Magainins as paradigm for the mode of action of pore forming polypeptides

Magainins are a class of antimicrobial peptides discovered in the skin of Xenopus laevis. The peptides kill bacteria by permeabilizing the cell membranes without exhibiting significant toxicity against mammalian cells, and are a promising candidate for a new antibiotic of therapeutic value. The main target of the peptides are considered to be the lipid matrix of the membranes. This review summarizes studies on magainin-lipid interactions in comparison with other pore forming peptides. The selective toxicity can be at least partly explained by preferential interactions of magainins with anionic phospholipids abundant in bacterial membranes. A novel mode of action is discussed in detail, i.e., the formation of a dynamic peptide-lipid supramolecular pore, which allows the mutually coupled transbilayer transport of ions, lipids, and peptides per se.

Identification and characterization of novel antimicrobial decapeptides generated by combinatorial chemistry

Novel combinatorial libraries consisting of simplified amino acid sequences were designed to screen for peptides active against the Candida albicans membrane. A novel decapeptide, KKVVFKVKFK, that had a unique primary amino acid sequence was identified in this work. This peptide irreversibly inhibited the growth of C. albicans and showed a broad range of antibacterial activity but no hemolytic activity. Circular dichroism spectra revealed that the predominant secondary structure of this peptide strongly depended on the membrane-mimetic environments; the peptide preferred to form an amphipathic alpha-helical structure in the presence of 50% trifluoroethanol, while it preferred to adopt a distorted alpha-helical structure in the presence of sodium dodecyl sulfate micelles. Experiments in which dye was released from vesicles indicated that this novel antimicrobial peptide killed microorganisms through the action on the membrane as its primary target. Replacement of amino acids in this active decapeptide on the basis of information from the libraries could provide unique information about factors affecting its antimicrobial activity such as its secondary structure, net positive charge, and hydrophobicity.

Structure-activity analysis of brevinin 1E amide, an antimicrobial peptide from Rana esculenta

Brevinin 1E, consisting of 24 amino acid residues, from Rana esculenta has potent antimicrobial and hemolytic activity. From a structural point of view, this peptide has a N-terminal hydrophobic region, a proline hinge region in the middle and a C-terminal loop region delineated by an intra-disulfide bridge, which is a common structural feature of antimicrobial peptides from Rana species. To investigate the structural features for antimicrobial and hemolytic activity, truncated and linearized brevinin 1E amides were synthesized and characterized. A deletion of three amino acids from the N-terminal region did not greatly affect antimicrobial activity but dramatically reduced hemolytic activity. The contribution of the intra-disulfide bridge to antimicrobial and hemolytic activity was somewhat different between brevinin 1E amide and truncated fragments. In brevinin 1E amide, the elimination of the intra-disulfide bridge did not greatly affect antimicrobial and hemolytic activity whereas the elimination of the intra-disulfide bridge in the truncated fragments did not decrease antimicrobial activity but did decrease hemolytic activity. Circular dichroism spectra and the retention time on the C18 reverse phase column revealed that the intra-disulfide bridge (i, i+6) formed an amphipathic loop which increased hydrophobicity and helped to induce the alpha-helical structure in the membrane-mimetic environment. Even though the intra-disulfide bridge and the N-terminal region were responsible for the alpha-helical structure and hydrophobicity, these two structural features were not essential for antimicrobial activity. The hemolytic activity of brevinin 1E amide and its analogs also correlated well with the retention time rather than the alpha-helicity.

Inhibitory action of a truncated derivative of the amphibian skin peptide dermaseptin s3 on Saccharomyces cerevisiae

The inhibitory activity of a truncated derivative of the natural amphibian skin peptide dermaseptin s3-(1-16)-NH2 [DS s3 (1-16)] against Saccharomyces cerevisiae was studied. Significant growth inhibition was observed after exposure to 3.45 microgram of the peptide per ml at pH 6.0 and 7.0, with complete growth inhibition occurring at 8.63 microgram of peptide per ml for all pH values tested. Using confocal scanning laser microscopy, we have shown that DS s3 (1-16) disrupted the yeast cell membrane resulting in the gross permeabilization of the cell to the nuclear stain ethidium bromide. However, the principal inhibitory action of the peptide was not due to disruption of intracellular pH homeostasis. Instead, growth inhibition by the peptide correlated with the efflux of important cellular constituents such as ADP, ATP, RNA, and DNA into the surrounding medium. The combination of DS s3 (1-16) with mild heating temperatures as low as 35 degreesC significantly enhanced the inhibitory effect of the peptide (8.63 microgram/ml), and at 45 degreesC greater than 99% of the population was killed in 10 min. In summary, a derivative of a natural antimicrobial peptide has potential, either alone or in combination with mild heating, to prevent the growth of or kill spoilage yeast.

Identification and characterization of the antimicrobial peptide corresponding to C-terminal beta-sheet domain of tenecin 1, an antibacterial protein of larvae of Tenebrio molitor

An active fragment was identified from tenecin 1, an antibacterial protein belonging to the insect defensin family, by synthesizing the peptides corresponding to the three regions of tenecin 1. Only the fragment corresponding to the C-terminal beta-sheet domain showed activity against fungi as well as Gram-positive and Gram-negative bacteria, whereas tenecin 1, the native protein, showed activity only against Gram-positive bacteria. CD spectra indicated that each fragment in a membrane-mimetic environment might adopt a secondary structure corresponding to its region in the protein. The leakage of dye from liposomes induced by this fragment suggested that this fragment acts on the membrane of pathogens as a primary mode of action. A comparison between the structure and the activity of each fragment indicated that a net positive charge was a prerequisite factor for activity. To the best of our knowledge this is the first report in which the fragment corresponding to the beta-sheet region in antibacterial proteins, which consists of alpha-helical and beta-sheet regions, has been identified as a primary active fragment.

Lipid and stress dependence of amphotericin B ion selective channels in sterol-free membranes

The idea that amphotericin B (AmB) may not require sterols to form ion selective channels has recently been criticized on the grounds that egg phospholipids commonly used in experiments may contain small amounts of sterol which associate with AmB to form AmB/sterol pore channel structures. It was recently shown in this laboratory that modest osmotic stress can enhance the formation of AmB channels in sterol-free egg phosphatidylcholine (eggPC) membranes. We have tested AmB's ability to form ion channels/defects in synthetic palmitoyl oleoyl (POPC), dieicosenyl (DEPC) and natural eggPC osmotically stressed large unilamellar vesicles (LUV) using pyranine fluorescence detected ion/H+ exchange. These sterol-free POPC LUV exhibit greatly increased sensitivity to cation selective AmB channel formation when osmotically stressed; even more than eggPC. Under these stressed conditions, AmB activity was observed at [AmB]/POPC ratios as low as 3.5x10(-4), corresponding to about 34 AmB molecules/vesicle. DEPC vesicles were almost completely unresponsive, demonstrating a strong bilayer thickness dependence. These results prove conclusively that AmB can form sterol-free channels and do so within therapeutic concentration ranges (>0.5-10x10(-6) M) in a stress-dependent manner. This phenomenon may allow us to use osmotic stress changes in simple model systems to spectroscopically isolate and characterize the thus-far elusive AmB channel forming aggregate. In addition, this stress dependence may be responsible for the potentiation of renal toxicity of AmB in the ascending branch of the loop of Henle which is under greatest osmotic stress.

Solution structure of a synthetic lytic peptide: the perforin amino terminus

BACKGROUND

Killer lymphocytes secrete perforin, a 67 kDa protein that initiates T-cell cytolysis following aggregation and pore formation in target membranes. The resulting pores cause a breakdown of the transmembrane osmotic gradient and allow other cytolytic mediators to enter the target cell and initiate apoptosis. The cytolytic domain resides within the first 34 residues of the amino terminus of perforin, with residues 1-19 being sufficient for cytolytic activity.

RESULTS

The solution structure of a 22-residue synthetic peptide (P22), corresponding to the amino terminus of human perforin, has been determined using high resolution nuclear magnetic resonance spectroscopy in the presence and absence of perdeuterated detergent (SDS) micelles. In aqueous solution, P22 exists mainly in a random conformation. However, it adopts a hook-like structure at the carboxyl terminus in the presence of SDS micelles when the positively charged residues cluster to form a turn that provides a binding surface to the negatively charged sulfate headgroups.

CONCLUSIONS

The strong electrostatic interaction between the cationic region of the P22 peptide and the lipid headgroups probably weakens the membrane, facilitating insertion of the relatively neutral/hydrophobic stretch of P22, and is representative of the initial step of the lytic pathway. The structural model described here is probably relevant to understanding the mechanisms of other cationic antimicrobial peptides.

The plasma membrane of Leishmania donovani promastigotes is the main target for CA(1-8)M(1-18), a synthetic cecropin A-melittin hybrid peptide

Reports on the lethal activity of animal antibiotic peptides have largely focused on bacterial rather than eukaryotic targets. In these, involvement of internal organelles as well as mechanisms different from those of prokaryotic cells have been described. CA(1-8)M(1-18) is a synthetic cecropin A-melittin hybrid peptide with leishmanicidal activity. Using Leishmania donovani promastigotes as a model system we have studied the mechanism of action of CA(1-8)M(1-18), its two parental peptides and two analogues. At micromolar concentration CA(1-8)M(1-18) induces a fast permeability to H+/OH-, collapse of membrane potential and morphological damage to the plasma membrane. Effects on other organelles are related to the loss of internal homeostasis of the parasite rather than to a direct effect of the peptide. Despite the fast kinetics of the process, the parasite is able to deactivate in part the effect of the peptide, as shown by the higher activity of the d-enantiomer of CA(1-8)M(1-18). Electrostatic interaction between the peptide and the promastigote membrane, the first event in the lethal sequence, is inhibited by polyanionic polysaccharides, including its own lipophosphoglycan. Thus, in common with bacteria, the action of CA(1-8)M(1-18) on Leishmania promastigotes has the same plasma membrane as target, but is unique in that different peptides show patterns of activity that resemble those observed on eukaryotic cells.

Interaction of alpha-helical peptides with phospholipid membrane: effects of chain length and hydrophobicity of peptides

To investigate the interaction of amphiphilic alpha-helical peptides with phospholipid membranes, we synthesized Ac-(Leu-Ala-Arg-Leu)3-NHCH3 (4[3]) and three derivatives, in which the chain length and the size of the hydrophobic region of the peptides were different from each other. These peptides formed an alpha-helical structure in the presence of vesicles. In the membrane-perturbation measurement, only 43 showed a strong membrane-perturbation activity below phase-transition temperature (25 degrees C), but above phase-transition temperature (50 degrees C), most peptides showed similar strong activities. On the other hand, in membrane-fusion measurement the long peptides, e.g., Ac-(Leu-Ala-Arg-Leu)3-(Leu-Arg-Ala-Leu)3-NHCH3, had strong activities at low peptide concentrations at 25 degrees C. The present study indicated a parallel relationship did not always exist between membrane fusion and perturbation caused by peptides.

Structure and dynamics of the antibiotic peptide PGLa in membranes by solution and solid-state nuclear magnetic resonance spectroscopy

PGLa, a 21-residue member of the magainin family of antibiotic peptides, is shown to be helical between residues 6 and 21 when associated with detergent micelles by multidimensional solution nuclear magnetic resonance (NMR) spectroscopy. Solid-state NMR experiments on specifically 15N-labeled peptides in oriented phospholipid bilayer samples show that the helix axis is parallel to the plane of the bilayers. 15N solid-state NMR powder pattern line shapes obtained on unoriented samples demonstrate that the amino-terminal residues are highly mobile and that the fluctuations of backbone sites decrease from Ala6 toward the carboxy terminus. The powder pattern observed for 15N-labeled Ala20 is essentially that expected for a rigid site. These findings are similar to those for the 23-residue magainin2 peptide in membrane environments.

Studies of kinetics and equilibrium membrane binding of class A and class L model amphipathic peptides

We studied the kinetics and equilibrium membrane binding of two amphipathic alpha-helical peptides: the 18L peptide, which belongs to the class L (lytic peptides), and the Ac-18A-NH2 peptide of the class A (apolipoprotein), according to classification of Segrest et al. ((1990) Proteins, 8, 103-117). Both for cationic 18L and zwitterionic Ac-18A-NH2, the presence of acidic lipids increased the membrane binding constants by two orders of magnitude. The free energy of peptide-membrane association was in the range of 8.5-12.8 kcal/mol. Binding isotherms corresponded to monomer partitioning with saturation at high peptide/lipid ratios. This was also supported by stopped flow studies of the kinetics of peptide-membrane association as measured by peptide tryptophan fluorescence or by energy transfer from the peptide to the lipid-anchored anthrylvinyl fluorophor. The apparent time required for peptide-membrane equilibration was in the millisecond range. At low peptide/lipid ratios it depended on lipid concentration and was independent of the peptide concentration. The rate of peptide-membrane association was found to be relatively close to the diffusion limit. Thus peptide-membrane affinity was mostly determined by the peptide dissociation rate, i.e. higher membrane affinity correlated with a decrease in dissociation rate and with a slower peptide exchange. We have shown that the dynamic character of the peptide membrane equilibrium can be used for selective peptide targeting and disruption of membranes with a specific lipid composition.

Antimicrobial activity and conformation of gaegurin-6 amide and its analogs

A function of the intra-disulfide bridge located at the C-terminal of Rana peptides has not been extensively studied. To investigate the function of the disulfide bridge related to the activity and the structure, we chose Gaegurin-6, isolated from Rana rugosa as a model peptide and synthesized linear analogs. The reduction of the disulfide bridge resulted in the complete loss of antimicrobial activity while replacements of cysteines by serines retained antimicrobial activity. Circular dichroism spectra from a titration of the peptides in sodium dodecyl sulfate indicated that the disulfide bridge of Gaegurin-6 might stabilize the induction of an alpha helical structure in lipid membranes and the alpha helical forming propensity of the peptides correlated with antimicrobial activity.

Design of synthetic antimicrobial peptides based on sequence analogy and amphipathicity

Novel alpha-helical antimicrobial peptides have been devised by comparing the N-terminal sequences of many of these peptides from insect, frog and mammalian families, extracting common features, and creating sequence templates with which to design active peptides. Determination of the most frequent amino acids in the first 20 positions for over 80 different natural sequences allowed the design of one peptide, while a further three were based on the comparison of the sequences of alpha-helical antimicrobial peptides derived from the mammalian cathelicidin family of precursors. These peptides were predicted to assume a highly amphipathic alpha-helical conformation, as indicated by high mean hydrophobic moments. In fact, circular dichroism experiments showed clear transitions from random coil in aqueous solution to an alpha-helical conformation on addition of trifluoroethanol. All four peptides displayed a potent antibacterial activity against selected gram-positive and gram-negative bacteria (minimum inhibitory concentrations in the range 1-8 microM), including some antibiotic resistant strains. Permeabilization of both the outer and cytoplasmic membranes of the gram-negative bacterium, Escherichia coli, by selected peptides was quite rapid and a dramatic drop in colony forming units was observed within 5 min in time-killing experiments. Permeabilization of the cytoplasmic membrane of the gram-positive bacterium, Staphylococcus aureus, was instead initially quite slow, gathering speed after 45 min, which corresponds to the time required for significant inactivation in time-killing studies. The cytotoxic activity of the peptides, determined on several normal and transformed cell lines, was generally low at values within the minimum inhibitory concentration range.

Modulation of membrane activity of amphipathic, antibacterial peptides by slight modifications of the hydrophobic moment

Starting from the sequences of magainin 2 analogs, peptides with slightly increased hydrophobic moment (mu) but retained other structural parameters were designed. Circular dichroism investigations revealed that all peptides adopt an alpha-helical conformation when bound to phospholipid vesicles. Analogs with increased mu were considerably more active in permeabilizing vesicles mainly composed of zwitterionic lipid. In addition, the antibacterial and hemolytic activities of these analogs were enhanced. Correlation of permeabilization and binding indicated that the activity increase is predominantly caused by an increased membrane affinity of the peptides due to strengthened hydrophobic interactions.

Dependence of the lytic activity of the N-terminal domain of human perforin on membrane lipid composition--implications for T-cell self-preservation

The kinetics and thermodynamics of pore formation by the 22-residue N-terminal domain of human perforin-(1-22)-peptide have been studied for a variety of model phospholipid membranes. Peptide binding and aggregation, and cell lysis were monitored through the change in the fluorescence of Trp, or vesicle-encapsulated carboxyfluorescein, respectively. Peptide binding was analyzed in terms of a model that incorporates non-ideal interactions and aggregation in a membrane bilayer. The minimum number of peptide monomers required to form an active pore averaged from four to six, according to the lipid composition of the vesicle. This combined kinetic and thermodynamic approach has provided quantitative information that allows a direct comparison of the binding behavior of the perforin-(1-22)-peptide in different lipid vesicles and affords molecular insight on the factors controlling pore formation. Pore formation is most favorable in thinner membranes with low melting temperatures. No significant difference in activity is observed for different zwitterionic headgroups. Rather, the gel state of the lipid chain, which diminishes the incorporation and aggregation of the perforin-(1-22)-peptide shows the strongest influence. This effect is observed in both the thermodynamic (incorporation isotherm) and kinetic (carboxyfluorescein release) studies. Insertion and aggregation are more facile in membranes with less densely packed lipids. The dependence of pore-forming activity on lipid composition provides important clues to understanding the self-protection mechanism employed by cytotoxic T lymphocytes (CTL) against perforin-mediated lysis.

Synthetic histatin analogues with broad-spectrum antimicrobial activity

Histatins are salivary histidine-rich cationic peptides, ranging from 7 to 38 amino acid residues in length, that exert a potent killing effect in vitro on Candida albicans. Starting from the C-terminal fungicidal domain of histatin 5 (residues 11-24, called dh-5) a number of substitution analogues were chemically synthesized to study the effect of amphipathicity of the peptide in helix conformation on candidacidal activity. Single substitutions in dh-5 at several positions did not have any effect on fungicidal activity. However, multi-site substituted analogues (dhvar1 and dhvar2) exhibited a 6-fold increased activity over dh-5. In addition, dhvar1 and dhvar2 inhibited the growth of the second most common yeast found in clinical isolates, Torulopsis glabrata, of oral- and non-oral pathogens such as Prevotella intermedia and Streptococcus mutans, and of a methicillin-resistant Staphylococcus aureus. In their broad-spectrum activity, dhvar1 and dhvar2 were comparable to magainins (PGLa and magainin 2), antimicrobial peptides of amphibian origin. Both the fungicidal and the haemolytic activities of dhvar1, dhvar2 and magainins increased at decreasing ionic strength.

Interactions of an antimicrobial peptide, magainin 2, with outer and inner membranes of Gram-negative bacteria

Magainin peptides, isolated from Xenopus skin, have broad spectra of antimicrobial activity and low toxicities to normal eukaryotic cells, thus being good candidates for therapeutic agents. The mechanism of action is considered to be the permeabilization of bacterial membranes. A number of studies using lipid vesicles have elucidated its molecular detail. However, their interactions with bacteria are not yet well understood. In this paper, we synthesized several magainin analogs with different charges (0 to +6) and hydrophobicities, and systematically studied their interactions with the outer and inner membranes of three species of Gram-negative bacteria (Escherichia coli, Acinetobacter calcoaceticus, Proteus vulgaris). The treatment of the E. coli cells with native magainin 2 (+4) immediately induced the efflux of the intracellular K+ ions and the cell death. A number of blebs were formed on the bacterial surface and the outer membrane became leaky. An increase in the peptide's positive charge enhanced the outer membrane permeabilization and the bactericidal activity. The cationic peptides also effectively permeabilized the inner membranes rich in acidic phospholipids, indicating the importance of electrostatic interactions. Substitution of Trp for Phe simultaneously increased the bactericidal activity and the hemolytic activity. A strategy to develop potent antimicrobial peptides was discussed on the basis of these results.

The effect of sterols on the sensitivity of membranes to the channel-forming antifungal antibiotic, syringomycin E

The ability of three sterols of different structure to influence the interaction of syringomycin E (an antifungal antibiotic that forms voltage dependent channels in planar lipid bilayers) with a planar lipid bilayer was evaluated. The rate of increase of bilayer conductance induced by syringomycin E was about 1000-times less in bilayers containing 50 mol% of cholesterol compared to bilayers without sterols. The effect of ergosterol (the primary sterol of fungal cells) on the sensitivity of bilayers to syringomycin E was much weaker than that of cholesterol, while stigmasterol (one of the main sterols of plant cells) did not significantly influence the ability of syringomycin E to induce a conductance increase in the bilayer. None of the sterols altered the single channel conductance properties of syringomycin E. These observations suggest that cholesterol affects the sensitivity of target membranes to syringomycin E by enlarging the energy barrier for channel formation rather than participating in channel formation itself.

Functional characterization of pediocin PA-1 binding to liposomes in the absence of a protein receptor and its relationship to a predicted tertiary structure

The physicochemical interaction of pediocin PA-1 with target membranes was characterized using lipid vesicles made from the total lipids extracted from Listeria monocytogenes. Pediocin PA-1 caused the time- and concentration-dependent release of entrapped carboxyfluorescein (CF) from the vesicles. The pediocin-induced CF efflux rates were higher under acidic conditions than under neutral and alkaline conditions and were dependent on both pediocin and lipid concentrations. A binding isotherm constructed on the basis of the Langmuir isotherm gave an apparent binding constant of 1.4 x 10(7) M-1 at pH 6.0. The imposition of a transmembrane potential (inside negative) increased the CF efflux rate by 88%. Pediocin PA-1 also permeablized synthetic vesicles composed only of phosphatidylcholine. Sequence alignments and secondary-structure predictions for the N terminus of pediocin PA-1 and other class IIa bacteriocins predicted that pediocin PA-1 contained two beta-sheets maintained in a hairpin conformation stabilized by a disulfide bridge. The structural model also revealed patches of positively charged residues, consistent with the argument that electrostatic interactions play an important role in the binding of pediocin PA-1 to the lipid vesicles. This study demonstrates that pediocin PA-1 can function in the absence of a protein receptor and provides a structural model consistent with these results.

Design of active analogues of a 15-residue peptide using D-optimal design, QSAR and a combinatorial search algorithm

This report describes the rational design of novel analogues of a 15-residue antibacterial peptide CAMEL0. A constrained D-optimal design was carried out to derive a training set of 60 analogues. Partial least squares (PLS) models describing quantitative structure-activity relationships (QSARs) were initially derived for the peptides using two published and one novel parameter set. The novel "Design parameters' were based on key structural features identified in hypothetical models of the mechanisms by which peptides interact with cell membranes. In an extension of the PLS method, influence statistics were used to decrease the weighting of compounds having a large effect on model predictions. A combinatorial search algorithm was developed which used PLS models as predictors to select a test set of 39 peptides with high predicted potencies. Within this set, the most potent analogue CAMEL135, which contained seven point mutations from CAMEL0, was identified. For a panel of 24 bacteria, the mean MIC value of CAMEL135 was approximately half of that for CAMEL0. For the parameter sets tested, covariance functions derived from Z-scales gave highest Q2-values for the training set, whilst the model using the the 'Design parameters' gave least error when predicting the activity of the test set. The predictive ability of a third published set of peptide parameters was found to compare favourably with that of the parameters used in the design. Analysis of the PLS models indicates that hydrophobicity and amphipathicity are the most important features influencing activity for this class of compound.

Effect of cholesterol and charge on pore formation in bilayer vesicles by a pH-sensitive peptide

The effect of cholesterol on the bilayer partitioning of the peptide GALA (WEAALAEALAEALAEHLAEALAEALEALAA) and its assembly into a pore in large unilamellar vesicles composed of neutral and negatively charged phospholipids has been determined. GALA undergoes a conformational change from a random coil to an amphipathic alpha-helix when the pH is reduced from 7.0 to 5.0, inducing at low pH leakage of contents from vesicles. Leakage from neutral or negatively charged vesicles at pH 5.0 was similar and could be adequately explained by the mathematical model (Parente, R. A., S. Nir, and F. C. Szoka, Jr., 1990. Mechanism of leakage of phospholipid vesicle contents induced by the peptide GALA. Biochemistry. 29:8720-8728) which assumed that GALA becomes incorporated into the vesicle bilayer and irreversibly aggregates to form a pore consisting of 10 +/- 2 peptides. Increasing cholesterol content in the membranes resulted in a reduced efficiency of the peptide to induce leakage. Part of the cholesterol effect was due to reduced binding of the peptide to cholesterol-containing membranes. An additional effect of cholesterol was to increase reversibility of surface aggregation of the peptide in the membrane. Results could be explained and predicted with a model that retains the same pore size, i.e., 10 +/- 2 peptides, but includes reversible aggregation of the monomers to form the pore. Resonance energy transfer experiments using fluorescently labeled peptides confirmed that the degree of reversibility of surface aggregation of GALA was significantly larger in cholesterol-containing liposomes, thus reducing the efficiency of pore formation.

Conformational and associative behaviours of the third helix of antennapedia homeodomain in membrane-mimetic environments

The third helix of antennapedia homeodomain pAntp-(43-58) can translocate through cell membrane and has been used as an intracellular vehicle for delivering peptides and oligonucleotides. The conformational and associative behaviour of two peptidic vectors pAntp-(43-58) and [Pro50] pAntp-(43-58) has been analyzed by different biophysical methods. pAntp-(43-58) adopts an amphipathic helical structure in 30% (by vol.) hexafluoroisopropanol, in perfluoro-tert-butanol and in the presence of SDS micelles. CD spectra indicate that the conformation of [Pro50]pAntp-(43-58) in contrast to pAntp-(43-58) is independent of the media used. 1H-NMR spectroscopy in SDS micelles or in perfluoro-tert-butanol allows detection of aggregated peptides probably in a ribbon 2(7) type conformation. These conformations became the predominant structure when Gln50 was replaced by Pro50. Interproton-distance restraints derived from NOE measurements have been classified in two groups corresponding to two types of structures: alpha-helix and essentially extended structures. Consecutive CH alpha (i)/ CH alpha (i + 1) NOEs are only compatible with aggregates. Simulated annealing calculation of dimeric structure agrees with phi and psi angles in the beta-sheet and gamma-turn regions. Fluorescence spectroscopy analysis has shown that the indole groups of both peptides penetrate into SDS micelles; both peptides also induce the formation of micelles at very low concentration of SDS (20 microM). Similar interaction was observed with reverse-phase micelles made of bis(2-ethyhexyl) sodium sulfosuccinate and small unilamellar vesicles (SUV) made of a mixture of phosphatidylcholine/phosphatidylserine. 31P-NMR of vesicles (SUV and large unilamellar vesicles) indicated that the addition of pAntp analogues did not affect the size of phosphatidylcholine/phosphatidylserine vesicles. The addition of pAntp analogues to lipidic dispersions modulates lipid polymorphism in different ways depending on the mixtures of acidic lipids.

Modulation of structure and antibacterial and hemolytic activity by ring size in cyclic gramicidin S analogs

We have evaluated the effect of ring size of gramicidin S analogs on secondary structure, lipid binding, lipid disruption, antibacterial and hemolytic activity. Cyclic analogs with ring sizes ranging from 4 to 14 residues were designed to maintain the amphipathic character as found in gramicidin S and synthesized by solid phase peptide synthesis. The secondary structure of these peptides showed a definite periodicity in beta-sheet content, with rings containing 6, 10, and 14 residues exhibiting beta-sheet structure, and rings containing 8 or 12 residues being largely disordered. Peptides containing 4 or 6 residues did not bind lipopolysaccharide, whereas longer peptides showed a trend of increasing binding affinity for lipopolysaccharide with increasing length. Destabilization of Escherichia coli outer membranes was only observed in peptides containing 10 or more residues. Peptides containing fewer than 10 residues were completely inactive and exhibited no hemolytic activity. The 10-residue peptide showed an activity profile similar to that of gramicidin S itself, with activity against Gram-positive and Gram-negative microorganisms as well as yeast, but also showed high hemolytic activity. Differential activities were obtained by increasing the size of the ring to either 12 or 14 residues. The 14-residue peptide showed no antibiotic activity but exhibited increased hemolytic activity. The 12-residue peptide lost activity against Gram-positive bacteria, retained activity against Gram-negative microorganisms and yeast, but displayed decreased hemolytic activity. Biological activities in the 12-residue peptide were optimized by a series of substitutions in residues comprising both hydrophobic and basic sites resulting in a peptide that exhibited activities comparable with gramicidin S against Gram-negative microorganisms and yeast but with substantially lower hemolytic activity. Compared with gramicidin S, the best analog showed a 10-fold improvement in antibiotic specificity for Gram-negative microorganisms and a 7-fold improvement in specificity for yeast over human erythrocytes as determined by a therapeutic index. These results indicate that it is possible to modulate structure and activities of cyclic gramicidin S analogs by varying ring sizes and further show the potential for developing clinically useful antibiotics based on gramicidin S.

The synthesis and characterization of analogs of the antimicrobial compound squalamine: 6 beta-hydroxy-3-aminosterols synthesized from hyodeoxycholic acid

Analogs of the aminosterol antimicrobial agent squalamine have been synthesized beginning from hyodeoxycholic acid. After carboxylic acid esterification and oxidation of both alcohol functions to ketones, the A/B ring junction was converted from cis to trans by acid-catalyzed isomerization. Different polyamines were added to the 3-keto group by reductive amination, yielding both the 3 alpha and 3 beta addition products. The synthetic products exhibited potent, broad-spectrum antimicrobial activity similar to that of the parent compound. Changing the identity of the polyamine or the stereochemistry of addition has little effect upon antimicrobial activity but appears to change the selectivity of the agents. The analogs are synthesized with high yield from inexpensive starting materials and are promising alternatives to squalamine as potential antibiotics.

A class of highly potent antibacterial peptides derived from pardaxin, a pore-forming peptide isolated from Moses sole fish Pardachirus marmoratus

Pardaxin, a 33-amino-acid pore-forming polypeptide toxin isolated from the Red Sea Moses sole Pardachirus marmoratus, has a helix-hinge-helix structure. This is a common structural motif found both in antibacterial peptides that can act selectively on bacterial membranes (e.g., cecropin), and in cytotoxic peptides that can lyse both mammalian and bacterial cells (e.g., melittin). Herein we show that pardaxin possesses a high antibacterial activity with a significantly reduced hemolytic activity towards human red blood cells (hRBC), compared with melittin. Its potency is comparable to that of other known native antibacterial peptides such as magainin, cecropins and dermaseptins. To determine the structural features responsible for the selective hemolytic and antibacterial activities, and the structural requirements for a high antibacterial activity, 8 truncated and modified pardaxin analogues were synthesized and structurally and functionally characterized. Each peptide was synthesized with a free carboxylate or amino group (i.e., aminated form) at its C-terminus. The aminated form of pardaxin has both high hemolytic and antibacterial activity. A truncated analogue, with 11 amino acids removed from the C-terminal domain, had dramatically reduced hemolytic activity. However, the aminated form of this analogue was significantly more potent that pardaxin against most bacteria tested, suggesting that the C-terminal tail of pardaxin is responsible for non-selective activity against erythrocytes and bacteria. Furthermore, a positive charge added to its N-terminus significantly increased its antibacterial activity and abolished its low hemolytic activity. The 22-amino-acid C-terminal domain and the short 11-amino-acid N-terminal domain were, in their aminated forms, active only against gram-positive bacteria. Secondary-structure determination using circular dichroism spectroscopy revealed that all the aminated analogues had 25-80% more alpha-helical content in 40% CF3CH2OH/water than their non-aminated forms. Using model phospholipid membranes it was found that all the analogues that were less hemolytic but had retained antibacterial activity could permeate acidicly charged phospholipid vesicles better than zwitterionic phospholipid vesicles, a property characteristics of all the native antibacterial peptides tested so far (e.g., cecropins, magainins and dermaseptins). Pardaxin and its analogues therefore represent a new class of antibacterial peptides that can serve as a basis for the design of therapeutic agents. Furthermore, negative-staining electron microscopy revealed that total inhibition of bacterial growth was due to total lysis of the bacterial wall. Therefore, it might be more difficult for bacteria to develop resistance to such a destructive mechanism, compared with the more specific mechanisms of the currently used antibiotics.

Broad spectrum antibiotic activity of the skin-PYY

Neuropeptide Y (NPY) and polypeptide YY (PYY) are two ubiquitous neuropeptides, found in brain and intestines, respectively, where they exert important regulatory functions. In this study, a new member of the YY family recently isolated from amphibian skin, skin-PYY (SPYY), is reported to inhibit irreversibly the proliferation of a broad spectrum of pathogenic microorganisms. NPY and PYY are shown to be endowed with the same activity. Their potency is similar to that of other antibacterial peptides which have been shown to exert their function by disintegrating the bacterial membrane. These findings and the fact that the C-terminal alpha-helical domain SPYY14-36, which is highly conserved among family members, was responsible for killing microorganisms and for permeation of phospholipid vesicles, suggested that the antibiotic activity may emerge via a membrane permeation mechanism. These findings also raise the question whether NPY and PYY exert in vivo a similar function in mammals.

Physiochemical characterization of the nisin-membrane interaction with liposomes derived from Listeria monocytogenes

Mechanistic information about the bacteriocin nisin was obtained by examining the efflux of 5(6)-carboxy-fluorescein from Listeria monocytogenes-derived liposomes. The initial leakage rate (percentage of efflux per minute) of the entrapped dye was dependent on both nisin and lipid concentrations. At all nisin concentrations tested, 5(6)-carboxyfluorescein efflux plateaued before all of the 5(6)-carboxyfluorescein was released (suggesting that pore formation was transient), but efflux resumed when more nisin was added. Isotherms for the binding of nisin to liposomes constructed on the basis of the Langmuir isotherm gave an apparent binding constant of 6.2 x 10(5)M(-1) at pH 6.0. The critical number of nisin molecules required to induce efflux from liposomes at pH 6.0 was approximately 7,000 molecules per liposome. The pH affected the 5(6)-carboxyfluorescein leakage rates, with higher pH values resulting in higher leakage rates. The increased leakage rate observed at higher pH values was not due to an increase in the binding affinity of the nisin molecules towards the liposomal membrane. Rather, the critical number of nisin molecules required to induce activity was decreased (approximately 1,000 nisin molecules per liposome at pH 7.0). These data are consistent with a poration mechanism in which the ionization state of histidine residues in nisin plays an important role in membrane permeabilization.